The present invention relates to material removing tools in general, and, more particularly, to improvements in rotary cutters. Still more particularly, the invention relates to rotary cutters of the type wherein a rotor is driven by a shaft and carries a plurality of external material removing bits.
Continuous miners by their nature utilize a series of rotating drums with cutter bits placed at very specific positions in order to accommodate a number of different cutting situations. There are two basic types of cutting drums. The ripper chain, which includes a central ripper chain between two rotating drums, and the solid head, which does not have a central ripper chain between the rotating drums.
The main advantage of the ripper chain is that when it cuts, it leaves no coring or gap in cutting profile between the rotating drums. This is because the power transmission webs for transferring power from a motor to the ripper chain and rotating drums is underneath the chain itself. The solid head doesn't have the ability to cover up the power transmission webs like a ripper chain does, so the area between the rotating drums must be left open for the power transmission webs. Because of this space left between the drums for the webs, that area can't be cut out by a cutting tool.
If the webs are too wide, the remaining uncut material will not break off naturally and can keep the machine from penetrating into the face of the material it's trying to cut. If the web is too narrow, then the webs don't have sufficient strength to support the drums and withstand the extreme forces the cutting head is subjected to. An optimum design is a web that's narrow enough to have the material core break off by itself and wide enough to withstand the cutting and fatigue loads it sees.
As shown in FIG. 1, as material is cut away from the face by the cutting tools 12, much of it is recirculated around the cutting drum 10. As this material is passed around the drum 10, the bit and the bit holders fracture and redirect the broken material to either side of the bit holder 12. This direction of material back to a structural web 16 causes impact and regrinding of the material against the web. This impacting of the material against the web erodes away the web steel regardless of its hardness and abrasion resistance. In even some instances, rock pieces can be wedged in between the bit holder 12 and the web 16 and cause catastrophic damage to any exposed components.
When possible, exposed tubes have been used to semi-protect the drive through shaft in the web from this type of damage. This isn't always possible, however, and warranty costs are incurred as a result. At the very least, the long term erosion that takes place causes a reduction in web strength. Expensive rework must be done periodically in order to get additional life from the rotating drums. As miner duty cycles increase, the web steel id less likely to make it through the complete rebuild cycle.